llvmpipe: only report supported shader-image formats
LLVMpipe has a rather limited implementation of shader-images, so let's limit what we report to the formats required by the spec, as all of those seems to work fine. Reviewed-By:Mike Blumenkrantz <michael.blumenkrantz@gmail.com> Reviewed-by:
Roland Scheidegger <sroland@vmware.com> Part-of: <mesa/mesa!10467>